Exam Notes: Electronic Structure of Atoms

Electronic Structure of the Atom

Introduction

• Bohr discovered that hydrogen absorbs energy and re-emits it as light when energy is added.
• This light, when passed through a prism, produces a line spectrum.

Bohr's Explanation

• Electrons in hydrogen exist only in specific energy states.
• These states correspond to specific circular orbits around the nucleus.
• Electrons move instantaneously between orbits when they absorb or emit energy.
• Higher energy levels correspond to orbits farther from the nucleus.

Energy Levels

• Energy Level: The specific amount of energy an electron can possess in an atom.
• Hydrogen energy levels follow a specific pattern, with n representing the energy level number.

Line Spectrum and Quantum Energy

• The line spectrum shows energy level differences when an electron loses energy and drops to a lower level.
• Only certain energy differences occur due to specific energy levels.
• Quantum of Energy: The difference between two distinct energy levels.

Refined Theory

• Electrons do not travel in circular orbits as Bohr initially proposed.
• Instead, electrons occupy specific regions of space called orbitals based on their energy.
• Orbital: A region of space where an electron is likely to be found, holding a maximum of 2 electrons.

Terminology

• Four main types of orbitals: s, p, d, and f.
• Shell: Orbitals with the same principal quantum number (n).
  • Example: The 3rd shell includes 3s, 3p, and 3d orbitals.
• Subshell: A set of orbitals of the same type.
  • Example: The 2p subshell contains three p orbitals.

Orbital Types and Shapes

s Orbitals

• Spherical shape.
• One s orbital per s subshell.
• Each s subshell can hold 2 electrons.

p Orbitals

• Shaped like balloons.
• Three p orbitals per p subshell.
• Each p subshell can hold 6 electrons.

d Orbitals

• Five d orbitals per d subshell.
• Each d subshell can hold 10 electrons.

f Orbitals

• Seven f orbitals per f subshell.
• Each f subshell can hold 14 electrons.

Important Notes

• For hydrogen, all shells with the same n value have the same energy.
• For a given n value, n different types of orbitals are possible:
  • n=1: only s-type orbitals
  • n=2: s- and p-type orbitals
  • n=3: s-, p-, and d-type orbitals
  • n=4: s-, p-, d-, and f-type orbitals

Energy Level Diagrams

• Diagrams showing the relative energy levels of orbitals in an atom or ion.
• Different diagrams for hydrogen (single-electron) and polyelectronic (multi-electron) atoms.

Energy Level Diagram for Polyelectronic Atoms

• Electron repulsion alters the energy levels of different orbitals.
• A specific energy level diagram applies to all atoms with more than one electron.

Electron Configurations

• Represent the occupied orbitals in an atom/ion and the number of electrons in each subshell.
• Governed by three rules:
  • Aufbau Principle: Electrons are added to the lowest energy orbitals first as the atomic number increases.
  • Pauli Exclusion Principle: Each orbital can hold a maximum of two electrons.
  • Hund's Rule: When electrons occupy subshells of equal energy, they singly occupy each orbital with parallel spins before pairing up with opposite spins.

Energy Level Diagram Practice

• Practice drawing energy level diagrams for various atoms (Si, Tc, Ca, Zr, Ga).
• A trick exists to determine orbital filling order without full electron configuration.

Writing Electron Configurations for Neutral Atoms

• Use the